Torbjörn Sjöstrand
Torbjörn Sjöstrand is a Swedish theoretical physicist and a professor at Lund University in Sweden, where he also got his PhD in 1982. He is one of the main authors of PYTHIA, a program for generation of high-energy physics events.
In his early career, Sjöstrand spent shorter postdoc periods at DESY and Fermilab. From 1989 to 1995 he was staff member in the CERN Theory division.
Scientific contributions
Sjöstrand's most enduring achievement is the Lund string model for hadronization, devised with Bengt‐Åke Andersson and others in the late 1970s and first implemented in the JETSET event generator. He later combined all the main steps of a high-energy particle collision—how quarks and gluons are first scattered, then multiply interact, and finally turn into the hadrons we detect—into one computer programme called PYTHIA. Today researchers at every major accelerator, from the old LEP and HERA rings to the Large Hadron Collider, rely on PYTHIA to simulate what their detectors should see. The current release, PYTHIA 8, cites more than 7,000 experimental and phenomenological papers and is downloaded about 25,000 times per year, according to the 2015 program overview in Computer Physics Communications.International roles
During a six-year stint in the CERN Theory Division he co-convened the LEP QCD Working Group and co-authored the handbook that set precision-tuning standards for event generators. Since 2006 he has been Swedish spokesperson for the EU-funded MCnet network, supervising joint schools that train around 120 PhD students annually in modern event simulation techniques.Publications and mentorship
Sjöstrand has authored more than 300 peer-reviewed papers and the textbook "Computational High-Energy Physics". He has supervised eleven PhD theses at Lund, including those of Torbjörn Sjögren and Christian Bierlich, both core PYTHIA developers.Honours
In 2012, he was awarded the J. J. Sakurai Prize for Theoretical Particle Physics by the American Physical Society. The citation reads:In 2020 Sjöstrand received Lund University's pedagogical prize for integrating open-source code labs into graduate theory courses. In 2021, he was awarded the High Energy and Particle Physics Prize of the European Physical Society for the development of PYTHIA. He received the award together with Bryan Webber, who was also a co-recipient of the Sakurai Prize.